A primary task of the heart is to pump oxygenated, nutrient-rich blood throughout the body. Electrical impulses generated by a portion of the heart regulate the pumping cycle. When the electrical impulses follow a regular and consistent pattern, the heart functions normally and the pumping of blood is optimized. When the electrical impulses of the heart are disrupted (i.e., cardiac arrhythmia), sudden cardiac arrest may result, which inhibits the circulation of blood. As a result, the brain and other critical organs are deprived of nutrients and oxygen. A person experiencing sudden cardiac arrest may suddenly lose consciousness and die shortly thereafter if left untreated.
A well known and effective treatment for sudden cardiac arrest or arrhythmia is defibrillation or cardioversion. Defibrillation involves passing a current through the person to shock the heart back into a normal rhythm. There are a wide variety of defibrillators. For example, implantable cardioverter-defibrillators (ICD) involve surgically implanting wire coils and a generator device within a person. ICDs are typically for people at high risk for a cardiac arrhythmia. When a cardiac arrhythmia is detected, a current is automatically passed through the heart of the user with little or no intervention by a third party.
Another, more common type of defibrillator is the automated external defibrillator (AED). Rather than being implanted, the AED is an external device used by a third party to resuscitate a person who has suffered from sudden cardiac arrest. FIG. 1 illustrates a conventional AED 100, which includes a base unit 102 and two pads 104. Sometimes paddles with handles are used instead of the pads 104. The pads 104 are connected to the base unit 102 using electrical cables 106.
A typical protocol for using the AED 100 is as follows. Initially, the person who has suffered from sudden cardiac arrest is placed on the floor. Clothing is removed to reveal the person's chest 108. The pads 104 are applied to appropriate locations on the chest 108, as illustrated in FIG. 1. The electrical system within the base unit 102 generates a high voltage between the two pads 104, which delivers an electrical shock to the person. Ideally, the shock restores a normal cardiac rhythm. In some cases, multiple shocks are required.
Although existing technologies work well, there are continuing efforts to improve the effectiveness, safety and usability of automatic external defibrillators. For example, if the electrical resistance of the patient can be lowered, then it would be possible to have a defibrillator that requires less power, but effectively delivers the therapeutic current to shock the heart.
Currently the main mechanism available to reduce the patient impedance is to increase the size of the electrode pads. The larger the surface area of the electrode pads, then the lower the patient impedance that the defibrillator faces. In addition these electrode pads make use of conductive hydrogel to help ensure that as much of each electrode is in conductive contact with the patient's skin as possible.
Properties of Human Skin
Human skin is the largest organ. Aside from the function of regulating skin temperature, the skin's most important function is to serve as an effective barrier against insult of the body by foreign agents, such as toxic substances, micro-organisms, and due to mechanical injury.
Skin is the outermost protective layer of the body. It is composed of the epidermis, including the stratum corneum, the stratum granulosum, the stratum spinosum, and the stratum basale, and the dermis, containing, among other things, the capillary layer. Below this is the subcutaneous fat layer. Above the skin, and growing outward from within the skin may be found hair, the strands of which can be up to 100 microns in thickness. Some “durable” skin layers such as heels or calluses, can comprise a stratum corneum which is from 100-150 microns thick.
The stratum corneum is a tough, scaly layer made of dead cell tissue. It consists of almost laminated layers of keratin from dead cells. It extends around 10-20 microns from the skin surface and has no blood supply. Because of the density of this layer of cells, moving electrical signals, electromagnetic energy or compounds across the skin, either into or out of the body, can be very difficult. Experiments have found the topmost layers of the stratum corneum to be the most resistant.
The epidermis is typically 50-150 microns in thickness and the dermis, which contains the capillaries and nerve endings is typically 750-1500 microns in thickness. Conductivity of the skin varies by a variety of conditions, such as age, location, sun exposure, use of lotions, moisture level, and ambient conditions, etc.
Removal of the stratum corneum reduces the high impedance of the skin and allows better transmission and reception of electrical signals, electromagnetic energy or biological species into and from human tissues. It has also been demonstrated that electromagnetic energy induced alterations of the stratum corneum result in increased permeability to substances. Alternatively, compounds commonly referred to as “permeation enhancers” can be used, with some success, to penetrate the stratum corneum. Traditional approaches require the abrasion of skin with sand paper and brushes, the stripping of skin with tape and toxic chemicals, the removal of stratum corneum by laser or thermal ablation, or the puncturing of skin with needles.
Thus, it is desirable to provide a better way to reduce patient transthoracic impedance for the purpose of delivering a therapeutic current, such as for a defibrillator or cardioverter, and it is to this end that the disclosure is directed.